In recent years, various ceramic electronic elements, such as a multilayer ceramic capacitor, have been widely mounted on various electronic devices, such as an automobile electronic control device.
As an electronic material used for this type of ceramic electronic element, a dielectric ceramic material primarily composed of barium titanate (BaTiO3) has been well known, and research and development therefor has been actively performed.
For example, in Patent Document 1, a dielectric ceramic composition has been proposed which contains BaO, TiO2, SnO2, Bi2O3, MgQ, and SiO2 in a predetermined composition range as an essential primary component and at least one of La2O3, Sm2O3, and Nd2O3 in a predetermined range as a selective essential component.
In this Patent Document 1, by controlling the composition of the essential primary components and the selective essential components in a predetermined range, a dielectric ceramic composition is obtained which has a relative dielectric constant εr of 1,000 or more and a dielectric loss tan δ of 10% or less and which is suitably used for a so-called delay line.
In addition, in Patent Document 2, a dielectric ceramic composition has been proposed which is primarily composed of a perovskite compound represented by a composition formula (Ba1-xSnx)mTiO3 in which 0.01≦x≦0.3 and 0.9≦m≦1.1 hold, and in which a Ba site represented by the (Ba1-xSnx) substantially includes no Sr.
In this Patent Document 2, Ba of BaTiO3 is partly replaced with Sn in a predetermined molar range, and the molar ratio of the Ba site to the Ti site is controlled in a predetermined molar range, so that a lead-free dielectric ceramic composition is obtained which has a Curie temperature Tc of 130° C. or more, a remanent polarization of approximately 20 μC/cm2 or more, and a high relative dielectric constant εr of 700 or more.
In general, since Sn in the form of a quadrivalent cation maintains its stable state, it is believed that in a BaTiO3-based compound, Sn is usually solid-solved in the Ti site. In addition, in Non-Patent Document 1, it has been disclosed that although the Curie temperature Tc of BaTiO3 is 120° C., when Ti is partly replaced with Sn, the Curie temperature Tc is significantly decreased from 120° C. to room temperature or less.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 3-040962
Patent Document 2: International Publication No. WO 2005/075377 Pamphlet
Non-Patent Document 1: K. Okazaki, “Ceramic Engineering for Dielectrics”, third edition, Gakken-sha Publishing Co., Ltd., June 1983, pp. 281 to 283
According to the dielectric ceramic composition disclosed in the Patent Document 1, since the Curie temperature Tc is low, such as −20 to +15° C., and the relative dielectric constant εr is rapidly decreased as the temperature is increased, there has been a problem in that this dielectric ceramic composition is difficult to be practically used in a high-temperature atmosphere. In particular, since a multilayer ceramic capacitor has been widely used for automobile applications in recent years, it has been requested to be used even in a high-temperature atmosphere at approximately 150° C.
Incidentally, the reason the dielectric ceramic composition disclosed in the Patent Document 1 has a low Curie temperature of −20 to +15° C. as described above is believed that in consideration of the description in the Non-Patent Document 1, Sn is solid-solved in the Ti site.
On the other hand, according to the dielectric ceramic composition disclosed in the Patent Document 2, a high Curie temperature Tc of 130° C. or more is obtained since Sn is solid-solved in the Ba site.
However, in the Patent Document 2, since an oxygen partial pressure in firing is extremely low, such as 10−12 to 1016 MPa, the degree of freedom of firing conditions is limited, and as a result, there has been a problem in that properties of a ceramic electronic element are difficult to be adjusted. In addition, for example, due to the change in firing temperature, the insulation properties of a dielectric ceramic may be degraded in some cases. Furthermore, when this dielectric ceramic composition is used for a multilayer ceramic capacitor, as the thickness of internal electrodes is decreased, a conductive material is sphered when ceramic layers and conductive films to be formed into the internal electrodes are co-fired, and as a result, so-called electrode breakage may disadvantageously occur in some cases.
That is, although quadrivalent Sn generally maintains a stable state, in order to solid-solve Sn in the Ba site, quadrivalent Sn must be formed into divalent Sn. Hence, in the Patent Document 2, a firing treatment is performed in a reducing atmosphere in which the oxygen partial pressure is extremely decreased to 10−12 to 10−16 MPa. However, on the other hand, since the oxygen partial pressure in firing is extremely low as described above, the degree of freedom of firing conditions is limited, the insulating properties of the dielectric ceramic are degraded, for example, by the change in firing temperature, and furthermore electrode breakage of the internal electrode may disadvantageously occur in some cases.